A conventional gas sensor is comprised of an outer electrode (detection electrode) formed on an outer surface of a closed-bottomed cylindrical base body which is made of solid electrolyte and an inner electrode (reference electrode) formed on an inner surface of the base body. Electromotive force generated in the solid electrolyte, which serves as the base body, is taken out from the outer electrode and the inner electrode to thereby detect oxygen concentration or the like.
There has been a demand that the outer electrode of such a gas sensor element is required not to be peeled off from a base material, even when the outer electrode is exposed for a long period of time to a high temperature gas to be detected. In order to fulfill such a demand, there have been proposed various methods, for example, a method wherein the outer surface of the base body is roughened by etching process or the like, and a method wherein a porous layer or projections is/are formed on the outer surface of the base body before or after sintering the base body.
For example, a conventionally known method for forming projections on the outer surface of the base body is that adhesive particles comprised of: granules more than half of which are occupied by particles having a particle size of 44 micrometers or more; and fine particles having a size of 10 micrometers or less, are applied to the base body, and thereafter they are simultaneously sintered. In this way, peeling of the outer electrode can be effectively prevented. See, for example, Patent Document 1, Japanese Patent Application Laid-Open (kokai) No. S56-160653.
Further, when such projections are formed, water vapor tends to enter a gap between neighboring projections and causes a phase transition of zirconia. The projections change in volume on a microscopic scale, causing the formation of minute cracks. As a result, the projections break away from the base body, causing a crack failure. In order to prevent such crack failure, it has been disclosed that the projections are formed into a single layer, the area occupied by the projections accounts for 10 to 85%, and a distance between the neighboring projections is 5 to 40 micrometers. See, for example, Patent Document 2, Japanese Patent Application Laid-Open (kokai) No. H11-230930.